A photonic integrated circuit (PIC) consists of many routed planar waveguides with multiple coupling ports, which need to be coupled to optical fibres. Aligning and attaching fibres to the ports on a PIC incur insertion losses due to Fresnel reflection and fibre-waveguide misalignment and these losses are one of the most important problems in applying PICs to optical telecommunications systems.
Edge-coupling, or butt-coupling as it is often referred to, is one of the most common methods of attaching fibres to a PIC. This method is widely used since the fibre can be conveniently mounted on the edges of a packing module. However, aligning the fibre to-the edge of a waveguide, or coupling port, without incurring excessive losses can be time consuming and difficult. The alignment process can be either passive or active. Passive alignment requires structures aligned to the coupling ports to be fabricated on the PIC or a substrate and the fibres mounted on the structures. Active alignment requires that the light emitting devices on the PIC are turned on and the fibre physically moved to enable maximum coupling or light into the fibre to be obtained. The fibre is then sealed in place using an adhesive or through laser welding.
On a PIC made up of only a few fibre attachments, fibre alignment and mounting issues are usually not significant a problem. However, for more complicated multiple optical port PICs, aligning each fibre individually to each coupling port incurs excessively high packaging costs. To overcome this problem, an array of V-grooves aligned to the coupling ports on the edges of a chip can be etched on the chip (or on a suitable substrate such as silicon) and the fibres mounted on them. However, this adds to the size and packaging complexity of the chip.
It is accepted that the number of fibres that need to he coupled to a PIC chip can be the main factor in determining its size. Only the edges or a PIC are available for fibre coupling, hence the number of coupling ports is limited to the length of the perimeter of the chip. Additionally, the need to mute the light to the edge for fibre attachment limits the flexibility of a PIC designer and can lead to unnecessarily long waveguides, which incur propagation losses and affect the device performance. Hence, edge coupling is not an efficient and cost effective method for multiple optical port coupling to a PIC, especially if the number of ports is large.
U.S. Pat. No. 5,195,150 and U.S. Pat. No. 4,948,960 disclose the use of a recess in the optical device substrate for receiving an optical fibre, thereby allowing vertical positioning of the fibre with respect to the planar device structure. However, due to the typical thickness of the device substance it is usually necessary to etch a deep recess in the substrate, thereby compromising the mechanical rigidity and structural integrity or the device. Furthermore, despite the depth of this type of recess, the remaining bulk substrate still presents a substantial propagation distance between the light emitting or guiding layers and the optical fibre. Consequently, and as described in U.S. Pat. No. 5,195,150, it is often necessary to fabricate an integrated lens to ensure good light collection and efficient coupling to the optical fibre. Such structures add further to the complexity of device fabrication.
Other techniques for vertical placement of an optical fibre with respect to a planar optical device have been proposed, but these often suffer from the problem of mismatch between planar waveguide and optical fibre dimensions. An example is the use of a fibre with an angled end facet, which necessitates thinning of the optical fibre to allow placement proximate to the underlying waveguide, resulting in a very brittle fibre tip.